Menu
Search
How the Weather Affects Conductors
How the Weather Affects Conductors (photo credit: Todd Klassy via Flickr)

Sag

Notice how a conductor is strung between poles. It is not pulled tight; it sags. In hot weather it sags even more than in cold weather. Consider the analogy of the clothesline.

A tightly strung clothesline as shown in Figure 1a puts a much greater strain on the poles which hold it at either end. Adding the weight of wet clothes to a tightly strung line might even pull the poles out of place. However, using a little more rope and allowing the clothesline to dip a little (see Figure 1b) would relieve this pull considerably.

This dip is what is called sag. The same applies to stringing wire.

When metallic wire is strung tightly, it produces a greater strain on the insulator pins and on the pole. Scientific sag is an important factor in stringing wire.

Illustration of sag. (a) A taut line causes strain on poles. (b) With a properly sagged line, poles can easily withstand strain.
Figure 1 – Illustration of sag. (a) A taut line causes strain on poles. (b) With a properly sagged line, poles can easily withstand strain.

A line sags more in hot weather and less in cold weather. The reason for this is because conductors expand in hot weather; in other words the length of the conductor increases as the temperature increases.

It follows that in cold weather the metallic conductor will be shorter than in warm weather.

If the wire were strung without sag, it would snap during cold weather. On the other hand, there is a chance a wire strung with too much sag would dip below the specified clearance in warm weather (Figure 2).

Conductor expansion. (a) Conductors shrink in cold weather and expand in hot weather. (b) The line must never sag below the minimum clearance. (c) The line must never be so tight that it strains the poles.
Figure 2 – Conductor expansion. (a) Conductors shrink in cold weather and expand in hot weather. (b) The line must never sag below the minimum clearance. (c) The line must never be so tight that it strains the poles.

For copper conductors, the change in length within a temperature range of 100′ to 0° is over 5 feet per 1000 feet; for aluminum the change is almost 7 feet.

Besides the temperature, there are other factors that must be considered in determining the sag of a conductor, for example:

  • the length of the span,
  • the weight of the conductors,
  • wind, and
  • ice loading.
Also, conductors must not touch each other when swaying in the wind.

Reference: Guide to Electrical Power Distribution Systems – Anthony J. Pansini (Get hardcopy from Amazon)

About Author //

author-pic

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

9 Comments


  1. Pongsak Harnboonyanon
    Jul 13, 2014

    Good article for basic understanding of sag of bare conductor.For the last two
    paragraphs I think the minimum height of conductor above ground and temperature rise of
    the conductors are also the factors that should be taken into account for determining sag..
    How do you calculate capacitance of sag lines in practice?


  2. RAVI RANJAN TIWARY
    Jul 09, 2014

    IT IS QUIET EASY TO UNDERSTAND COMPARE OF ALL HARD COPIES WITH PICTORIAL WAY..
    VERY USEFUL.


  3. saas
    Jul 06, 2014

    how can i download this articles in pdf format from EEP?
    thank you


    • RAVI RANJAN TIWARY
      Jul 09, 2014

      YOU JUST CLICK BY MOUSE, RIGHT CLICK AND ADOPT OPTION SAVE AS AND SAVE PDF IN UR DISK.


  4. Pinank Nagda
    Jul 06, 2014

    As sag increases wont the capacitance of the line change(decreases). This will affect the line capacitance and hence the bus admittance matrix in the power flow equations. So power flow controller would have to take these calculations in mind while dispatching of power else the line may get damaged.


  5. saurabh yadav
    Jul 04, 2014

    lacks in detailing of the topic….


  6. saurabh yadav
    Jul 04, 2014

    well the article lacks in detailing, the details provided in the article can be understood by common scene..


  7. Debraj Chatterjee
    Jul 04, 2014

    please make an application of EEp for windows operated phones..


  8. Debraj Chatterjee
    Jul 04, 2014

    how can i download this articles in pdf format from EEP?

Leave a Comment

Tell us what you're thinking... we care about your opinion!


Get PDF